Histogenese und morphologische Deutung der Blätter von Psilotum nudum

Abstract

1. The sterile leaf primordia originate at the growing point from three original cells at the surface layer, one placed upon the other as seen in longitudinal section. Subsequently these cells divide peri — and anticlinally. From the upper cell the ventral side of the leaf primordium is formed; the middle cell divides mainly periclinally and — in some cases — forms only a single cell row in the middle part, whereas from the lower cell the dorsal side of the leaf primordium originates, including the leaf base. 2. Owing to the strong development of the dorsal side the primordium bends from the beginning hyponastically over the growing point. 3. The sterile leaf primordia show neither an apical nor marginal growth and growth in thickness – as realized in higher plants – is likewise absent 4. The bifurcation of the fertile leaf is brought about by cessation of the growth in the middle portion of the leaf top, while the lateral regions continue in their growth. As this growth consists mainly in periclinal division of surface cells and underlying cells, it can possibly be interpreted as a primitive form of a beginning marginal growh. 5. The edges of the stem originate from periclinal divisions of superficial cells whereby radial cell rows are formed in the cortex. Possibly, this stimulation of the meristem formation in the axis is induced in an acropetal direction by older leaves. 6. As seen from the histological point of view, the leaves of Psilotum nudum are to be considered as rudimentary. Concerning the phylogenetic aspects, Psilotum cannot be interpreted as a primitive type, but as a reduced one. Vergrößerung aller Abbildungen 6×42, soweit nichts anderes vermerkt.